In some engineering applications structural components are affected by the fluids they are in contact with. Example applications include all manner of chemical process equipment and piping systems, water treatment and distribution systems (industrial water and potable water systems, boiler systems and cooler systems, etc.) and oil and gas pipelines for collection, processing and distribution. In many of these applications, it is advantageous to monitor damage accumulation, predict component life, and control fluid properties to minimize damage to system structural components. Damage to structural components from fluids can include corrosion, erosion, scale and/or oxidation. Many structural components can be difficult to inspect, can be hidden from observation, can cause health and environmental damage in the event of failure, and/or can be costly to maintain. Advanced sensors are needed to actively monitor the physical effects of fluids in contact with structural components and minimize their deleterious effects. For example, physical effects sensors can be used as feedback in control systems for the injection of green treatment chemicals and corrosion inhibitors to control corrosion, biological growth, scaling in water treatment, chemical process and boiler systems. Advanced physical effects monitoring, such as corrosion, erosion, scale and/or oxidation, will result in reduced maintenance costs, increased component service life and safer operations.
A limited range of measurement technologies are conventionally utilized to determine the rate, type and physical effects that damage structural components, such as corrosion. These technologies can be grouped into two general categories, 1) metal loss and 2) electrochemical methods.
Metal loss measurement methods include mass loss coupons and electrical resistance devices. These techniques provide cumulative corrosion and possibly erosion, and corrosion rate data. Typically, the mass loss coupons are placed into the process stream or a side stream that can be accessed without process interruption. The coupons can be made of an alloy that is the same as the structural component being monitored, or can be a standard material including steels, stainless steel, copper and brass. Mass loss coupons are considered to be reliable for measuring corrosion over longer time periods at discrete intervals. To quantify corrosion, the mass loss coupons are retrieved cleaned and weighed. Mass loss coupons can be used in nearly any process, but do not allow for continuous monitoring, are labor intensive and require significant space within the process system or structure being monitored.
Electrical resistance sensors can continuously monitor cumulative corrosion and possibly erosion and corrosion rate of metal elements. The principle of operation of resistance sensors is measurement of ohmic losses as the cross section of the sensing element decreases due to corrosion. The resolution and service life of the resistance probes vary based on sensor thickness with the highest resolution achieved at the expense of sensor life. The more sensitive resistance probes have response times of about 100 hours for corrosion rates of 1 MPY. Resolution is reduced by thermoelectric voltages and electromagnetic noise. Resistance probes provide for continuous monitoring without process interruption and function in virtually any environment, except molten metal and conductive molten salts. The metal loss methods provide data in environments where fouling occurs, although the fouling may effect the corrosion rate and mode of attack (under deposit corrosion).
Electrochemical methods, including Linear Polarization Resistance (LPR), Electrochemical Impedance Spectroscopy (EIS) and Electrochemical Noise (EN) are used to monitor corrosion. These measurement techniques are used to quantify the thermodynamics and kinetics of electrochemical reactions associated with corrosion. LPR is used extensively to detect instantaneous corrosion rates, but application is limited to conductive solutions and performance is restricted in low conductivity waters and non-aqueous environments. Like resistance sensors, the resolution of electrochemical methods is reduced by thermal and electromagnetic noise. Unlike resistance and mass loss coupon methods, electrochemical techniques are unable to detect metal loss due to erosion, or to provide a direct measure of cumulative material loss. Furthermore, electrode fouling limits the use of LPR and if fouling does occur the sensors are removed, cleaned and possibly returned to service. Electrochemical methods may require specialist knowledge for data interpretation.
A variety of fiber-optic sensors based on Extrinsic Fabry-Perot Interferometric (EFPI) technology are known as disclosed in U.S. Pat. Nos. 5,301,001; 6,341,185; 6,426,796 and 6,571,639, the entire content of each such prior-issued patent being expressly incorporated hereinto by reference. In general, the Extrinsic Fabry-Perot Interferometric (EFPI) technology measures distance based on a low-finesse Fabry-Perot cavity formed between the polished end face of a fiber and a reflective surface.
There exists a need in the art, therefore, for reliable sensors that may be used for the continuous detection of fluid effects on structures and/or components in contact with the fluid. Further, there is a need for physical effects sensors that may have long service life, that may detect cumulative damage and damage rates, and may be compatible with automated monitoring and control systems. It is towards fulfilling such needs that the present invention is directed.